Lower -crf may give higher quality but for screen captures the improvement is only marginal. For post processing in iMovie or Final Cut Express better to use .mov container in the above conversion.

If ffmpeg complains that it has no hq preset, then try some of the others. Look for .ffpreset files in /usr/share/ffmpeg/. I had good success with -vpre medium (actually it might be the same as hq used to be).